Concrete is widely used in contact with water in constructions such as piers, bridge pillars, oil platforms etc. Concrete may also be used to make anchors for releasably tethering a submarine device at the seabed. Submarine devices are used for many purposes, for example, Sea Bed Logging surveys. These surveys require measuring devices to be tethered on the seabed, remain static during the survey, and be released afterwards so that the expensive device can be reused.
The measuring device, to the top of which a floater element is attached, is strapped to a concrete anchor element. The anchor then helps to sink the device in a stable manner and to secure a stable position on the seabed. After the measurements are finished, the device is released and floats to the surface leaving the concrete anchor behind. The concrete anchor is left on the seafloor and apart from the fact that it is a foreign object on the seabed, it may subsequently present an obstacle for fisheries (e.g. trawling) or other industrial activity.
Therefore, it would be desirable to develop concrete that will disintegrate within a limited time after contact with water, and, for seawater applications, preferably only in seawater. In order to prevent the concrete anchors forming obstacles for trawling and other activities, the concrete should disintegrate shortly after the end of the useful life of the anchor. A secondary advantage of such an approach would be to ensure recovery of the expensive measuring devices after some time in cases where the release mechanism should fail.
The concrete composition should disintegrate into components that are not harmful to the environment and marine life.
The hydraulic binder of concrete based on Portland cement is amorphous calcium silicate hydrate (CSH-gel) where some 25% crystalline calcium hydroxide is embedded. Other less abundant minerals exist as well.
If sufficient calcium carbonate is added to such a concrete (e.g. as limestone filler), it is known that the concrete will be prone to degradation by sulphate attack at low temperatures (<15° C.), even if a so called sulphate resistant Portland cement is used. The binder will actually crumble and turn into a mush since CSH gel is transformed to Thaumasite (a calcium silicate carbonate sulphate hydrate; Ca3Si(OH)6(CO3)(SO4).12H2O) without binding properties. Three components are required to form Thaumasite:
1. Calcium silicate (taken from the cement paste)
2. Calcium carbonate (e.g. addition of limestone filler)
3. Sulphate (usually intruded from the surroundings)
The formation of Thaumasite is discussed by Sibbick, T., Fenn, D. and Crammond, N. in “The Occurrence of Thaumasite as a product of Seawater Attack”, Cement and Concrete Composites, Vol. 25, No. 8, December 2003, pp. 1059-1066. The bedding mortar of a recently constructed harbour wall step in South Wales had suffered severe cracking and spalling within 2 years. The reaction products formed included Thaumasite, Ettringite, Brucite and hydrated magnesium silicate. The study proved that concrete with limestone will eventually form Thaumasite in line with the chemical changes outlined above.
This reference discusses the undesired formation of Thaumasite and the problems caused thereby. However, the aim of the current invention is to provide a concrete formulation which may be used for seabed anchors, which will cause the anchor to disintegrate substantially shortly after the end of the useful life of the anchor. The useful life of the anchor after deployment in the sea is of the order of 1 month.